Optimizing sensitivity and resolution of ion chamber



July 26, 1960 OPTIMIZING SENSITIVITY AND RESOLUTION OF' ION CHAMBER Filed Jan. 30, 1957 J. G. CASTLE, JR 2,946,887

3 Sheets-Sheet 1v l I C. a,

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ATTORNEY .5

July 26, 1960 J, G, CASTLE, 1R 2,946,887

OPTIMIZING SENSITIVITY AND RESOLUTION OF ION CHAMBER Filed Jan. 30, 1957 V 3 Sheets-Sheet 2 INVENTOR.

July 26, 1.960 .1. G. CASTLE, JR 2,946,887

OPTIMIZING SENSITIVITY AND RESOLUTION OF ION CHAMBER Filed Jan. 30, 1957 .'5 Sheets-Sheet 3 INVENTOR. Eff/f 6. snf

Tam/5V segment 18 is added to the inner cylinder at the input side of the ion chamber and is concentrically surrounded by a cylindrical reference electrode 20. This additional segment 18 andY its associated concentric cylinder 20 constitute what will be herein referred to as an ion filter.` The dimensions of the filter and collector electrodes `are determinable from formulae hereinafter provided. Electrode 20 is connected to the source of D.C.,potential 3,67, the inner segment 18 of the filter being grounded. AsY may be noted, an electric eld exists in the space between electrodes 18 and 20 but not between electrodes 10 and 20 which are both at the same D.C. potentials.

Figs. 2 and 4 show elevational andside view crosssections of the parallel-plate type of ion chamber. Top i and bottom electrodes 23 and 22, as well as the inner electrodes 24, 26 and28 and the lter components 29, 30, 31 and 32 are supported Vby the side pieces 35 and 34 which are composed of insulating material such as polystyrene. The inner collecting electrodes 24, 26 and 28 and the collecting electrode segment of the lter may be hollow rectangular boxes having flat upper and lower surfaces, as shown, or simply llat rectangular plates. The supply voltage wiring can be fed through Vthe hollow space between the inner electrodes in both types of ion chamber.

The trajectory of an ion in a coaxial chamber may be visualized with the aid of Figure 1. The high mobility ions are collected near x=0. All the ions faster than a f.

certain cutoff mobility, kc, are collected on the length 0 to x. Ions entering near the upper electrode and having the cutol mobility -for x=x2 would follow the dashed lines in Figures l and 2 labelled kcz.

The value of the cutoff mobility, kc, 'for a particular length x is given in general by the expression zifomv (l) where Another mobility is significant when a filter Vis used, namely, the slowestmobility, ks, for which any ions are collected; These trajectories are marked ksl and ksg in Figures l and 2. The specific expressions are given below for coaxial and for at plate geometry.

In Figure l the trajectory of an ionY of mobility k for laminar flow in a coaxial chamber is where U is air velocity along x, and

b and a are radii of outer and inner electrodes, respectively.

The x=0 is estimated to be the midpoint of the rst insulator; it is actually-determined by the effective length corresponding to the measured capacity of the chamber. T he cutoff mobility, kc, is in turn given from either Equation (1) or Equation (2') for any specified x as 4 The fraction of the ions of mobility k entering the chamber which is collected on the length 0 to x2 is:

without filtering. For-a complete lter from r0=0 to r0=f, the length 0 to x2 picks up no ions slower than ks, given by the expression f a 2121952 U1n(b/a) For such a lter the fraction of the ions of mobility k entering the chamber (in front of the filter) which is For optimum filtering the slowest mobility -for which any ions are collected on section 1 should be the same as the cut-o: mobility for section 2. That is, the optimum filter, as discussed above, has ks1=kca ks2=kca, ctc. The fractional collection on length 0 to x2 is with this filter,

Another important factor to be considered is the length of the filter. Since the length of the filter must be finite, it is impossible to filter all the ions from the air entering the filter portion. For an incomplete filter having a cut-olf mobility kf, ions slower than kf will get through ata rate of k/ kf. It is found that the optimum length of the filter section is kf=kc3=ks2, for a three section chamber.

With a filter of this length the fractional collection rate in the third section is closely The fractional collection for the individual Asections "of collecting electrode is shown graphicallyl in Figure 3, for

a three section chamber having a resolution factor-of three. The sharpening effect of the filter appears as the difference between section 2 and section 3.

In Figure 2, the trajectory of an ion for laminar ilow between at plates is a straight line since the electric field for the configuration inwhich the Width of the electrodes is much larger than their separation, d. Equations 4, 6, 7 and 8 apply to the flat plate chamber as well as to the coaxial configuration.

From the above it will be seen that, to provide the maximum effective filtering with the minimum loss in sensitivity, the filter must receive that fraction of the total air entering the chamber which is the reciprocal of the factor of mobility (resolution factor). The filter should also be long enough so that its cutoff mobility is at least equal to that of the last section of the chamber, in order to realize the optimum advantages of the filter. The ion chamber used as an example has a mobility factor of three, and would require that 17% of the incoming air be filtered. Thus, for a three-section coaxial ion chamber having an outer-electrode'radius of b and an inner-electrode radius of a, the radius of the filter electrode (designated f in Fig. 1) should be bz an the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. A multiple-section ion chamber formed with a passage through which ion-bearing Iiiuid may flow comprising a filter section at the entrance to said passage and aV plurality of arranged serially collecting sections, the cumulative lengths of said collecting sections bearing the relationship nin-1 wherein x represents distance along the length of said collecting sections from the plane of juncture of said filter section and the first collecting section, n represents the number of collecting sections and m represents the resolution factor for the desired ion mobility ranges to be detected by the collecting sections, the area of said filter section comprising a proportion substantially equal to l/ m of the total area of the entrance to said passage.

2. The method of classifying a gas as to the mobility of the charged particles contained therein comprising passing the gas at an even rate through a sectional iondetection chamber, collecting in each section of said chamber only those particles having a mobility equal to the mobility of those collected in the next succeeding section divided by a preselected fixed numerical factor, and filtering the ions from a portion of the entering gas 4substantially equal to the reciprocal of said factor.

3. The method of classifying a gas as to the'mobility of the charged particles suspended therein comprising passing the gas at an even rate through a multiplesectioned ion chamber wherein the mobility of particles collected in each section is equal to the mobility of those collected in the next succeeding section divided by a preselected fixed numerical factor, and filtering from a portion of the gas before it enters the chamber yall particles having a mobility equal to or greater than that of those to be collected, the proportion of gas filtered being substantially equal to the reciprocal of said numerical factor.

4. The method of optimizing the resolution and sensitivity of an ion chamber wherein a gas is passed at an even rate through a plurality of collecting sections having cumulative lengths bearing the relationship x represents distance along the length of said collecting sections from the plane of juncture of said lter section and the first collecting sectiornn represents the number of collecting sections and m represents the resolution factor for the desired ion mobili-ty ranges to be detected by the collecting sections, and wherein the cutoff mobility of the last section is Ken, comprising ltering from a pro-` portion of said gas equal to l/ m all ions having a mobility greater than Ken.

5. An ion chamber for use in classifying a gas as to the mobility of charged particles contained therein comprising a pair of relatively displaced plates of electrically conductive material adapted to be maintained at a fixed electrostatic potential relative to each other, one of said plates being longitudinally divided into a filter segmentY and a plurality of collecting segments all insulated from each other, the lengths of said collecting segments being such that the cumulative lengths of the collecting segments from the end of the filter segment adjacent the collecting segments varies in accordance with the following series m0, m1, m2, m3 mnl, wherein m represents the resolution factor for the desired ion mobility range to be detected by the collectingsegments and n represents the number of collecting segments in the chamber, and a third plate of electrically conductive material having a length substantially equal to that of said filter segment so positioned between said pair of plates adjacent ysaid filter segment that the area of the entrance to said chamber between said filter segment and said third plate is equal to the reciprocal of said resolution factor times the total entrance area.

6.7A multiple-section ion-detection chamber formed with a passage through which ion-bearing iiuid can flow comprising a filter section and a number of collecting sections yelectrically insulated from each other and arranged serially along the length of said passage, the cumulative lengths of said collecting sections varying in accordance with the relationship wherein x represents distance along the length of said collecting sections from the plane of juncture of said filter section and the first collecting section, n represents'the number of collecting sections and m represents the resolution factor for the desired mobility ranges to be detected by the collecting sections, the open area of said filter section transverse to the distance x being substantially equal to l/m of the total transverse open area at the entrance to said passage, and the length of said filter section being sufficient to provide said filter section with a cutoff mobility characteristic at least equal to that of the last collecting section along said passage.

7. A multiple-section ion-detecting chamber formed with a passage through which ion-bearing fiuid can flow comprising: an outer cylinder of electrically conductive material; an inner cylinder of electrically conductive material, said inner cylinder being coaxial with said outer cylinder and being divided longitudinally into a number of segments electrically insulated from each other, said innerrand outer sections being adapted to maintain an electrostatic field therebetween; a middle cylinder of elec` trically conductive material coaxial with and substantially equal in length to the `first segment at the entrance to the chamber, said first segment and said middle cylinder forming a filter section and the remainder of said inner segments and said outer cylinder forming ion-collecting sections, the cumulative lengths of said collecting-section segments varying in accordance with the relationship wherein x represents distance along the length of said collecting-section segments from the plane of juncture of said filter section and the first collecting section adjacent thereto, n represents the number of collecting sections and m represents the resolution factor for the desired mobility ranges to be detected by the collecting sections, the open area of said filter section transverse to the distance x being substantially equal to l/m of the totaltransverse open area at the entrance to said passage, and the length of said filter section being suicient to provide said filter with a cutol mobility characteristic at least equal to that of the last collecting section along said passage. l

References Cited in the le of this patent UNITED STATES PATENTS 

